Link adaptation in general packet radio service networks

ABSTRACT

In one embodiment of the method of adaptively selecting an airlink coding scheme in a telecommunications network, a coding scheme operating region is determined based on a currently used coding scheme and measurements representative of a block error rate. A block error coding scheme is determined based on the determined coding scheme operating region. In another embodiment of the method of adaptively selecting an airlink coding scheme in a telecommunications network, a first coding scheme is determined based on measurements representative of one or more conditional channel quality metrics, and a second coding scheme is determined based on measurements representative of a block error rate. One of the first and second coding schemes is selected as the coding scheme.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to General Packet Radio ServiceNetworks; and more particularly, link adaptation in such networks.

[0003] 2.Description of Related Art

[0004] To exploit the wide range of carrier-to-interference (C/I) ratiosexperienced over the radio interface, General Packet Radio Service(GPRS) networks employ four different airlink coding schemes. An airlinkcoding scheme is an error correction code that generates a total of nencoded bits for each k information bits, and therefore, has a codingrate $\frac{k}{n}.$

[0005] CS-1 is GPRS's lowest rate code. CS-4 is GPRS's highest ratecode. In the set containing codes {CS-1, CS-2, CS-3}, CS-3 is thehighest rate code, CS-1 is the lowest rate code.

[0006] One coding scheme is stronger than another if it is capable ofcorrecting more channel errors per block. CS-1 is stronger than CS-2,CS-3 and CS-4. Similarly, CS-2 is weaker than CS-1, since CS-2 iscapable of correcting fewer channel errors than CS-1.

[0007] GPRS's lowest rate code, CS-1, employs a relatively high numberof redundancy bits and offers a maximum logical link control (LLC)-layerthroughput of 8 kbps/timeslot. The high level of redundancy present inblocks encoded using CS-1 ensures that mobile stations at the fringes ofa cell, where C/I levels are typically lowest, are able to send andreceive data. In contrast, the highest rate code, CS-4, offers maximumLLC-layer throughputs of 20 kbps/timeslot. Because of the small numberof redundancy bits added to each block encoded with CS-4, however,airlink errors can be detected, but not corrected. As a result, CS-4offers the best airlink performance at relatively high C/I ratios. Theremaining two GPRS coding schemes offer maximum LLC-layer throughputs of12 kbps/timeslot (CS-2) and 14.4 kbps/timeslot (CS-3).

[0008] By monitoring the quality of the airlink, the GPRS network canselect the coding scheme that offers the best performance. The processof dynamically selecting the coding scheme based on airlink quality iscalled link adaptation.

[0009] A link adaptation algorithm, for example, may wish to select thecoding scheme that maximizes link layer throughput, subject to blockerror rates falling below a desired maximum target (˜20%). Such aperformance measure strikes a balance between the goal of airlinkefficiency (high throughputs) with the goal of keeping delay variancetolerable (low block error rates).

[0010] However, link adaptation decisions based on maximizing throughputalone neglects the debilitating effects high block error rates have onhigher layer protocols. High block error rates can cause a block to beretransmitted several times before it is correctly received. Highlyvariable delay in the transmission of radio link control (RLC) blockscan cause RLC flow control windows to stall, throttling the rate atwhich data can be delivered between the GPRS network and a mobile.Variable delays also wreak havoc on connection-oriented protocols suchas TCP, triggering retransmissions which waste airlink resources. Highdelay variance is also annoying to end users.

SUMMARY OF THE INVENTION

[0011] In the link adaptation methodology according to one embodiment ofthe present invention, a coding scheme is determined based onmeasurements indicative of block error rate. Specifically, a codingscheme operating region is determined based on the measurements, andcoding scheme corresponding to the determined coding scheme operationregion is selected. To determine the coding scheme operation region, oneembodiment of the present invention estimates a block error rate for apredetermined one of the coding schemes in each coding scheme operatingregion based on the measurement. Based on a comparison of the estimatedblock error rates to expected block error rates for the predeterminedcoding scheme in each coding scheme area, the coding scheme operatingregion is determined. In another embodiment, region indicators for eachcoding scheme operating region are generated based on the measurements,and the coding scheme is determined based on the generated regionindicators.

[0012] According to a further link adaptation methodology of the presentinvention, a coding scheme is selected from a coding scheme determinedbased on measurements indicative of block error rate and a coding schemedetermined from one or more conditional channel quality metrics such asbit error rate. In one embodiment the strongest coding scheme betweenthe two coding schemes is selected. In another embodiment, the codingscheme selected is based on the block error rate. A low block error rateindicates, for example, that a bit error rate based coding scheme isappropriate, while a high block error rate indicates that the bit errorrate coding scheme is not necessarily appropriate.

[0013] As a result of the link adaptation methodologies according to thepresent invention, link adaptation decisions do not neglect thedebilitating effects high block error rates have on higher layerprotocols.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, wherein like referencenumerals designate corresponding parts in the various drawings, andwherein:

[0015]FIG. 1 illustrates an example of a portion of a General PacketRadio Service (GPRS) network;

[0016]FIG. 2 shows the difference between the perceived mean Bit ErrorRate (BER)—the BER estimated by counting the number of bit errors whichoccur in successfully decoded blocks—and the true channel BER for thepropagation model TU3FH;

[0017]FIG. 3 shows a sample observation interval;

[0018]FIG. 4 shows an idealized plot of the block error rates fordifferent coding schemes as a function of C/I for a propagationenvironment;

[0019]FIG. 5 shows the Coding Scheme (CS) that will be used as a resultof an aging procedure according to the present invention;

[0020]FIG. 6 shows a high-level architecture for a downlink linkadaptation feature;

[0021]FIG. 7 shows a high-level architecture for an uplink linkadaptation feature; and

[0022]FIG. 8 illustrates the loose coupling between closed-loop powercontrol and link adaptation according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0023]FIG. 1 illustrates an example of a portion of a General PacketRadio Service (GPRS) network. As shown, a base station controller (BSC)10 communicates with one or more base transceiver stations (BTS) 12 overA-bis interface 14. The BTSs 12 communicate in a wireless fashion withmobile stations, such as mobile station 16, in their respective cells(not shown for the purposes of clarity).

[0024] The BSC 10 includes a packet control unit (PCU) 18 that preparescontrol and information data for transmission by the BTSs 12 to mobilestations according to the well-known GPRS RLC protocol. The PCU 18 alsoprocesses data received from the mobile stations via the basetransceiver stations 12 according to the GPRS RLC protocol. Morespecifically, the PCU 18 performs the link adaptation methodology of thepresent invention. This involves selecting the downlink coding schemebased on channel quality measurements received from the mobile station16, and selecting an uplink coding scheme, which is sent as controlinformation to the mobile station, based on channel quality measurementsby the BTS 12.

[0025] To support link adaptation, RLC/MAC layer of GPRS providesmechanisms for mobile stations to report downlink quality measurements.Downlink channel quality reports provide the GPRS network withinformation it needs to select appropriate coding schemes to use in thedownlink for each mobile station, and to revise the decision as channelquality changes. The RLC/MAC layer also provides mechanisms for thenetwork to command each mobile to use specific coding schemes for uplinktransmissions.

[0026] Downlink Link Adaptation

[0027] During packet transfer mode, the mobile station assesses downlinkchannel quality. Table 1 summarizes the channel quality measurementsreported by the mobile station in messages such as the Packet DownlinkACK/NACK message. TABLE 1 Summary of downlink quality measurementsreported by Mobile Stations. Reported Value Description C_VALUE 6-bitlong binary encoding of “C”, the estimated normalized received signallevel of the broadcast control channel (BCCH). The range is [−110, −48]dBm. RXQUAL_i When CS-1, CS-2, or CS-3 are used on the (i = 0, . . . ,7) downhnk, mobile stations report one of eight possible valuesspecifying the estimated bit error rate (BER) of the downlink channel.The BER estimate is calculated over successfully decoded blocks receivedduring the measurement interval. RXQUAL_0 = BER <0.2% RXQUAL_i =(0.1*2^(I))% < BER < (0.1*2^(i+1))% (i = 1, . . ., 6) RXQUAL_7 = 12.8% <BER When CS-4 is used on the downlink, mobile stations are permitted toreport RX_QUAL = 7, regardless of the quality of the channel. SIGN_VAREstimated variance of the received signal strength calculated overcorrectly decoded blocks. The measured signal variance is reported as a6 bit binary value. If (SIQN_VAR)₁₀ denotes the decimal value of the 6bit number, reported SIGN_VAR levels have the following meaning:(SIGN_VAR)₁₀ = 0 0 dB² to 0.25 dB² (SIGN_VAR)₁₀ = i [0.25*i]dB² to[0.25*(i + 1)]dB² (i = 1, . . . , 62) (SIGN_VAR)₁₀ = 63 >15.755 dB²I_LEVEL_TNi Estimated received downlink interference power on (1 = 0, .. . , 7) timeslot i, measured over search and PTCCH frames. The measuredinterference level is reported as a four bit binary value (0000→I_LEVEL0, . . . , 1111→I_LEVEL 15). Reported I_LEVELS on each timeslot have thefollowing meaning: I_LEVEL 0 interference level is greater than CI_LEVEL i C − (i + 1)*2 dB < interference level ≦ C − i*2 dB (I = 1, . .. , 14) I_LEVEL 15 interference level ≦ C − 30 dB

[0028] The measurements reported by mobile stations can be used toassess the quality of the downlink channel:

[0029] C_VALUE: The C_VALUE reported by the mobile station can be usedto assess the mean attenuation of the downlink signal received by amobile station. BCCH_LEVEL-C_VALUE is the average path-loss (in dBm) ofthe downlink channel. C_VALUE is used as a criterion in base stationselection and handover.

[0030] RX_QUAL: When CS-1, CS-2 or CS-3 are used, RX_QUAL is the mobilestation's estimate of the mean bit error rate of the downlink channelsince the last channel quality report was sent. A mobile station's biterror rate estimiates are constructed by averaging the number of biterrors observed in correctly decoded blocks addressed to it. Hence,RX_QUAL is a biased estimator of the actual downlink bit error rate. BERis fairly insensitive to channel propagation effects. RX_QUAL is a goodindicator of which coding scheme should be used when block error ratesare low.

[0031] When CS-4 is used on the downlink, a mobile station is allowed toreport RX_QUAL-7, regardless of the bit error rate of the channel As aresult, RX_QUAL will not be a meaningful measure of link quality whenCS-4 is used.

[0032] SIGN_VAR: SIGN_VAR measures the variance in received signalstrength over the four bursts comprising a radio block. High values ofSIGN_VAR are indicative of high mobile station speeds, while low valuesof SIGN_VAR are indicative of slowly moving mobile stations. Frequencyhopping is likely to result in higher values of SIGN_VAR when comparedto the case when frequency hopping is not present. Rayleigh fadingcauses variation in the signal strength levels over the four burstscomprising a single block. At low mobile speeds, attenuation levelsexperienced by each burst are highly correlated. Hence, the variance inreceived power over successive bursts that comprise a block at lowmobile speeds are low. As a result of the correlation in signalstrength, bit errors tend to be highly correlated at low mobile stationspeeds. At higher mobile station speeds, however, the signal levels ofthe four bursts that comprise a block exhibit high variance. At higherspeeds, channel bit errors are not as highly correlated as they are atlow speeds.

[0033] Under similar mean C/I and shadowing conditions, when frequencyhopping is not used, block error rates for CS-1, CS-2 and CS-3 tend tobe higher for slower moving mobile stations than they are for fastermobile stations. Hence, SIGN_VAR can provide potentially usefulinformation for link adaptation and power control algorithms.

[0034] I_LEVEL_TNi. Mobile stations make interference measurementsduring two search and two packet timing control channel (PTCCH) frameswhich occur during each 52-multiframe. These four “measurementopportunities” occur once every 60 ms. A mobile station must perform atleast N_(AVG) _(—) _(I) measurements on a timeslot before it can reportvalid interference measurements for the timeslot. (N_(AVG) _(—) _(I) isa tunable system parameter ranging from 1 to 180.) If a validinterference measurement is not available for a timeslot at the time amobile station sends a channel quality report, no interferencemeasurements are reported for that timeslot.

[0035] Uplink Link Adaptation

[0036] Uplink blocks received by the a BTS 12 are forwarded to the PCU18 over the A-bis interface 14. Each uplink block is encapsulated in aPCU frame. The PCU frame's header includes fields that encode thequality of the received block.

[0037] BFI If the BTS 12 is unable to correctly decode a block, it canset a Bad Frame Indicator (BFI) bit in the PCU frame to 1. A BFI valueof 0 indicates that the BTS was able to correctly decode the block.

[0038] RXLEV The BTS 12 indicates the average received power of the fourbursts comprising the block. RXLEV can be encoded using a total of sixbits as follows:

[0039] RXLEV 0=less than −110 dBm

[0040] RXLEV 1=−110 dBm to −109 dBm

[0041] RXLEV 2=−108 dBm to −107 dBm

[0042] . . .

[0043] RXLEV 62=−49 dBm to −48 dBm

[0044] RXLEV 63=greater than −48 dBm

[0045] RXQUAL For correctly decoded blocks, the BTS can determine thenumber of bits that were in error in the received radio block. Thenumber of bit errors in each block gives useful information on thequality of the data link.

[0046] Each uplink GPRS block contains a total of 456 bits. The BTScalculates the bit error rate (BER) as (number of bits inerror)/456*100%. The BER can be encoded using only 3 bits of the PCUframe header. A suitable encoding is as follows: RXQUAL BER Number ofvalue range bits in error RXQUAL 0 BER < 0.2% 0 RXQUAL 1 0.2% < BER < 10.4% RXQUAL 2 0.4% <BER < 2-3 0.8% RXQUAL 3 0.8% < BER < 4-7 1.6% RXQUAL4 1.6% < BER <  8-14 3.2% RXQUAL 5 3.2% < BER < 15-29 6.4% RXQUAL 6 6.4%< BER < 30-58 12.8% RXQUAL 7 BER > 12.8%  59-456

[0047] Selecting a Code Based on BER

[0048] Channel bit error rate (BER) is an excellent predictor of codingscheme block error rates. As such, it is an attractive metric to use forlink adaptation. Because of the large number of bits per airlink block(456 bits/block), BER can be measured accurately over a relatively smallnumber of radio blocks (˜5). Rapid, accurate channel quality estimationis crucial to properly selecting a coding scheme for use over atime-varying airlink.

[0049] The BTS and mobile, however, cannot always accurately estimatethe channel bit error rate. Channel bit error rate estimates areinherently biased, since they are obtained by averaging the number ofbit errors observed in correctly decoded blocks. When block error ratesare relatively low, bit error rates made in this fashion are close tothe actual bit error rate. At higher block error rates, the estimate isunreliable, resulting in underestimation of the true bit error rate ofthe channel.

[0050]FIG. 2 shows the difference between the perceived mean BER—the BERestimated by counting the number of bit errors which occur insuccessfully decoded blocks—and the true channel BER for the propagationmodel TU3 with frequency hopping. For example, at C/I's of 12 dB orbetter, the perceived bit error rate curves for CS-1 and CS-2 closelytrack the actual channel bit error rate. At C/I levels in excess of 12dB, block error rates are less than 10% for coding schemes CS-1 andCS-2. When block error rates are low, perceived bit error rate estimates(RX_QUAL) give useful information about which coding scheme should beused over the airlink.

[0051] Link adaptation based on perceived BER is straightforward: codingschemes are selected using a network-operator tuneable table mappingperceived bit error rate (RX_QUAL) to the maximal rate coding schemethat should be used. A coding scheme selected in this manner will bedenoted CS_(BER). When block error rates are high (>10%), however, linkadaptation decisions made on the basis of perceived BER will always endup selecting a weaker code than what is actually needed. Also, becauseRX_QUAL values are unreliable when CS-4 is used over the interface, theapproach can not be used when CS-4 is used to encode airlink blocks.

[0052] Selecting a Code Based on Observed Block Errors

[0053] A link adaptation algorithm monitors channel quality over anobservation interval, during which channel conditions are assumed to berelatively static. Based on what occurs during the observation interval,the link adaptation algorithm decides which coding scheme to use forfuture blocks.

[0054]FIG. 3 shows a sample observation interval. Because ofretransmissions and changes in coding scheme decisions made over time,the observation interval may contain blocks sent using different codingschemes. Because of the relatively high fraction of block errors duringthe observation interval and the presence of CS-4 blocks, perceived biterror rate-based link adaptation decisions as described above can beunreliable.

[0055]FIG. 4 shows an idealized plot of the block error rates fordifferent coding schemes as a function of C/I for a propagationenvironment.

[0056] As shown in FIG. 4, the operational C/I range is divided intofour disjoint regions. “Region i” denotes the C/I range in which codingscheme CS-i should be used. Let the block error rate for a CS-4 radioblock in region i be denoted P_(i). Suppose that under the same channelcondition in region i the probability that a CS-j RLC block is receivedin error is or α_(i,j)P_(i). Assume that the constants α_(i,j) are knownand independent of the propagation model. Although this assumption isnot entirely accurate, it is only required that α_(i,j) be of the sameorder in magnitude. This is because in practice it is extremely unlikelyto have more than 20 blocks in any given observation interval. As aresult, small differences in α_(i,j) will not affect the outcome of ourmethod. Besides, we are not really interested in estimating the trueblock error rate, but only in deciding which of the 4 coding schemes canbe supported under the current channel conditions. This, too, allows themethod to be fairly insensitive to the choice of α_(i,j). That α_(i,j)is of the same order in magnitude for various propagation models hasbeen validated through simulation results.

[0057] Under the assumption that the probability that a CS-j RLC blockis received in error is α_(i,j)P_(i) and that the channel lies in regioni, the value of P_(i) can be estimated from the observed block errorsover an observation interval. To show this, let N_(j) denote the numberof RLC blocks encoded using CS-j, which were received in the currentobservation interval, and let e_(j) denote the number of RLC blocks outof N_(j) which were in error. Accordingly, e_(j)/N_(j) represent theblock error rate (BLER). The mobile station reports e_(j) and N_(j) tothe PCU 18 for downlink adaptation purposes and the PCU 18 measurese_(j) and N_(j) for uplink adaptation purposes.

[0058] Assume that block errors are independent over the observationinterval. This assumption is justified provided:

[0059] Fluctuations in link quality caused by shadow fading andinterference power fluctuations are negligible over the observationinterval.

[0060] Rayleigh fading does not cause block errors to be correlated.This assumption is valid in systems employing frequency hopping. Insystems not employing frequency hopping, however, Rayleigh fading at lowmobile speeds will indeed cause block errors to be correlated whenblocks are sent to a mobile once every 20 ms. In general, thiscorrelation will cause the selection of a coding scheme to beconservative. And, the block error rate estimation approach disclosedherein will still work well on non-frequency hopped channels.

[0061] Under these assumptions, the probability P_(obs) of observinge_(j) blocks errors when a total of N_(j) blocks using coding schemeCS-j are sent over the observation interval (j=1, . . . , 4) is$\begin{matrix}{P_{obs} = {\prod\limits_{j}^{\quad}\quad {\begin{pmatrix}N_{j} \\e_{j}\end{pmatrix}\left( {\alpha_{i,\quad j}q} \right)^{e_{j}}\left( {1 - {\alpha_{i,\quad j}q}} \right)^{N_{j} - e_{j}},}}} & {{Equation}\quad 1}\end{matrix}$

[0062] where q is a variable denoting the (unknown) block error rate(BLER) for CS-4.

[0063] The maximum-likelihood estimate {circumflex over (P)}_(i) ofP_(i) given the sequence of block errors appearing in the observationinterval is given by $\begin{matrix}{{\hat{P}}_{i} = {\max\limits_{q}{\prod\limits_{j}^{\quad}\quad {\left( {\alpha_{i,\quad j}q} \right)^{e_{j}}\left( {1 - {\alpha_{i,\quad j}q}} \right)^{N_{j} - e_{j}}}}}} & {{Equation}\quad 2}\end{matrix}$

[0064] Therefore, {circumflex over (P)}_(i) is given as the solution tothe equation

g _(i)(q)=0,   Equation 3

[0065] where taking the log of equation 2 and differentiating gives:$\begin{matrix}{{g_{i}(q)} = {\sum\limits_{j}^{\quad}\quad {\left\lbrack {\frac{e_{j}}{q} - \frac{\left( {N_{j} - e_{j}} \right)\alpha_{i,\quad j}}{1 - {\alpha_{i,\quad j}q}}} \right\rbrack \quad.}}} & {{Equation}\quad 4}\end{matrix}$

[0066] The quantity P_(i) characterizes the quality of the channel.Hence a set of switching thresholds T₁,T₂ and T₃ (1≧T₁>T₂>T₃≧0) aredefined to determine when different coding schemes may be used:

[0067] use CS-1, if P_(i)>T₁,

[0068] use CS-2, if T₂<P_(i)≦T₁,

[0069] use CS-3, if T₃<P_(i)≦T₂, and

[0070] use CS-4, otherwise.

[0071] Alternatively, CS-i should be used only if the true value ofP_(i) is less than some threshold T_(i-1). (It is assumed that T₀<1.)

[0072] Whether the condition {circumflex over (P)}_(i)<T_(i) is true iseasily determined through the following observation. The functiong_(i)(q) defined in Equation 4 is a decreasing function of q. So, itholds that

g _(i)({circumflex over (P)} _(i))>g _(i)(T_(i)), if {circumflex over(P)} _(i)<T_(i), and

g _(i)({circumflex over (P)} _(i))≦g _(i)(T _(i)) otherwise.   Equation5

[0073] Since g_(i)({circumflex over (P)}_(i))=0, (see Equation 3), itfollows that

g _(i)(T _(i))<0, if {circumflex over (P)}_(i)<T_(i), and

g _(i)(T _(i))≧0, otherwise.   Equation 6

[0074] Hence, the sequence of block errors in the measurement intervalshow CS-i can be used only if g_(i)(T_(i))<0.

[0075] Sequential hypothesis testing can be used to determine CS_(BLER),the maximal rate coding scheme that should be used on the air interfacebased on the block errors observed during a measurement interval.Pseudo-code for the sequential hypothesis testing algorithm performed bythe PCU 18 is shown below: $\begin{matrix}{{i = 1};\quad {{{while}\left( {{g_{i}\left( T_{i} \right)} < {0\quad {AND}\quad i} < 4} \right)}\left\{ {\left. i\leftarrow{i + 1} \right.;} \right\}}} \\{{{CS}_{BLER} = i};}\end{matrix}$

[0076] An alternative formulation of the functions g_(i)(·) is asfollows. Let B_(i,j) denote the block error rate for blocks sent usingcoding scheme j at the C/I corresponding to threshold T_(i). That is,

B_(i,j)=α_(i,j)T_(i)=α_(i,j)B_(i,4).

[0077] Hence,${g_{i}\left( T_{i} \right)} = {\sum\limits_{j}^{\quad}\quad {\left\lbrack {\frac{e_{j}}{B_{i,\quad 4}} - \frac{\left( {N_{j} - e_{j}} \right)B_{i,\quad j}}{\left( {1 - B_{i,\quad j}} \right)B_{i,\quad 4}}} \right\rbrack.}}$

[0078] The tuneable parameters of the functions g_(i)(·) as expressed inEquation 7 are a bit more appealing than the in Equation 4: they are theblock error rates expected when each of the four coding schemes are usedat each of the three switching thresholds. Instead of defining theswitching points in terms of the parameters T_(i) and α_(i,j), theswitching thresholds are defined in terms of the block error ratesexpected to see when switching from one coding scheme to another. LetBLER_(i) ^(CS-j) denote the expected block error rate for CS-j blockswhen a switch should be made from coding scheme CS-i to CS-(i+1). Theswitching threshold can then be expressed in terms of the vector {rightarrow over (BLER)}_(i)=(BLER_(i) ^(CS-1),BLER_(i) ^(CS-2),BLER_(i)^(CS-3),BLER_(i) ^(CS-4)). A total of three such vectors are defined bythe network operator: one for the block error expected when switchingfrom CS-1 to CS-2 ({right arrow over (BLER)}₁), one for the block errorsexpected when switching from CS-2 to CS-3 ({right arrow over (BLER)}₂),and one for the block error expected when switching from CS-3 to CS-4({right arrow over (BLER)}₃).

[0079] The vectors selected should satisfy the following constraints:

[0080] BLER_(i) ^(CS-j)≦BLER_(i) ^(CS-(j+1)), ∀i,j. This constraintcharacterizes the property that under identical channel conditions,weaker codes have higher block error rates than stronger ones.BLER_(i)^(CS − j) ≥ BLER_(i + 1)^(CS − j),

[0081] ∀i,j: This constraint characterizes the property that the blockerror rate of a coding scheme does not worsen as channel conditionsimprove.

[0082] Initial simulation results show that these vectors can be easilytuned, based on the fact that in the region where coding scheme CS-ishould be used, the block error rates for CS-j (j<i) should be nearly 0.

[0083] Since the channel quality is time-varying, the observationinterval must be fairly short (on the order of one second, or so). As aresult, an observation interval will contain on the order of ten blocksor so—far too few blocks to estimate block error rates smaller than 10%.For this reason, coding scheme decisions based on measured block errorrates will only be reliable when block error rates are relatively high.When block error rates are low, this scheme may end up selecting acoding scheme that is too weak to use on the channel. In an alternativeembodiment, the selection of the coding scheme according to the BLER isconceptually based on more than one coding scheme region map as shown inFIG. 4; wherein the region map used to determine the coding scheme isselected based on the desired quality of service and/or type of service.

[0084] Combining the BER-Based and BLER-Based Estimates

[0085] Let CS_(MAX) denote the highest rate code that should be usedgiven the airlink quality during the observation interval. The linkadaptation algorithm run by the PCU 18 will select CS_(MAX) as follows:

CS _(MAX)=min{CS_(BLER) ,CS _(BER)},   Equation 8

[0086] where the min{CS-x,CS-y} operator denotes the strongest of thetwo codes CS-x and CS-y. (For example, min{CS-2,CS-4}=CS-2.) Similarly,the max{CS-x, CS-y} operator will be used to denote the weakest of thetwo codes CS-x and CS-y. (For example, max{CS-2,CS4}=CS-4.)Additionally, instead of CS_(BER), the selection of Equation 8 isbetween CS_(BLER) and a CS based on any conditional channel qualitymetric such as BER, a function of the bit-wise soft decisionmetricsgenerated by an equalizer, variance in bit error rates, etc.Also, instead of having a look-up table mapping one conditional channelmetric to a coding scheme, a look-up table that uses multipleconditional channel metrics as inputs to map to a coding scheme can beused.

[0087] As a still further alternative, multiple look-up table can beused; wherein one of the look-up tables is selected based on the desiredquality of service, type of service and/or the block error rate.

[0088] Selecting a coding scheme in this manner overcomes thedeficiencies of CS_(BER) and CS_(BLER):

[0089] When block error rates are high, CS_(BLER) gives a reliableestimate of the coding scheme with the best performance, while CS_(BER)will most likely be too weak for the current channel conditions. In thisscenario, CS_(max) will be equal to CS_(BER).

[0090] When block error rates are low, CS_(BER) gives a reliableestimate of the coding scheme with the best performance while CS_(BLER)will most likely be too weak for the current channel conditions. In thisscenario, CS_(max) will be equal to CS_(BER).

[0091] Link-Quality Caching

[0092] Data transfer for GPRS applications may tend to be“bursty”—several packets sent over a relatively short period of time.For such applications, the period of time between packets may be longenough for a downlink Temporary Block Flow (TBF) to be torn down. TBFsfor mobiles running such applications will be short-lived. However, oncea TBF is torn down, another will likely be set up again soon after.Path-loss, shadowing, and interference conditions in the wirelessenvironment tend to be highly correlated over short periods of time (onthe order of one second). As a result of this high correlation, adownlink TBF with mobile station m beginning a short period of timeafter its previous downlink TBF on the same timeslot should experiencesimilar airlink quality. This is also true for uplink TBFs. The linkadaptation algorithm takes advantage of this correlation.

[0093] At the end of a TBF, the algorithm (i.e., the PCU 18) will storethe value of the coding scheme currently in use and other channelparameters. The cached values will be used for subsequent TBFs, suitablyaged.

[0094] The PCU 18 uses the same coding scheme used at the end of theprevious TBF (CS-x) provided a subsequent TBF begins within T_(cache)time units. Starting the TBF with the coding scheme max(CS-(x-1),CS-1)when the elapsed time falls between T_(cache) and 2T_(cache), and so on.

[0095] The separate cached information will be maintained for thedownlink and the uplink for the same mobile.

[0096] Aging Coding Schemes

[0097] The time intervals between blocks transferred to/from a mobilecan be long. For example, if the mobile has low-priority, or if thetimeslot is heavily loaded, then it is very likely that a significantamount of time will have elapsed since the last channel quality reportwas received from the mobile. In such cases, it is possible that theairlink quality being experienced by the mobile has changedsignificantly due to variations in interference, path loss and shadowfading. A higher BLER could be experienced as time passes, making itdangerous to use the same coding scheme for long periods of time withoutfrequent updates of channel quality. This problem is certainly a biggerconcern for weak coding schemes like CS-4.

[0098] In order to address this concern, a tuneable parameter CS_(age)is defined, and it is assumed that all coding schemes weaker thanCS_(age) are subject to this problem. Let CS_(DL) denote the codingscheme selected at the time of the last coding scheme update. IfCS_(DL)>CS_(age), then the PCU 18 ages CS_(DLADV) at a rate determinedby the tuneable parameter T_(age). This aging procedure continues untilthe aged CS equals CS_(age). The following example illustrates thismechanism. Assume that CS_(age)=2, and T_(age)=1 second (the units forthe parameter T_(age) is frames; however, for the purposes ofillustration only, the units of T_(age) have been shown as seconds).FIG. 5 shows the CS that will be used as a result of this agingprocedure.

[0099] In FIG. 5, the mobile sends a downlink packet containing achannel quality report, at time 0. Based on this report, the linkadaptation algorithm determines CS_(DL)=4. Suppose that due to heavyloading, downlink RLC blocks are scheduled for the mobile veryinfrequently. As a result of the caching procedure if an RLC block issent in the time interval [0, 1.0), then CS-4 will be used. After thattime, CS_(DL) will be aged; thus, CS-3 will be used in the interval[1.0, 2.0). The CS will be aged further after 2.0 seconds, and CS-2 willbe used after that. Since CS_(age)=2, the CS will not be aged anyfurther, and CS-2 will continue to be used until the link adaptationalgorithm updates CS_(DL). The next channel quality report is receivedat time=4. The link adaptation algorithm recommends the use of CS-3. So,CS-3 will be used for the next 1 second, after which CS-2 will be used(because of the aging procedure). This process continues for theduration of the TBF.

[0100] The aging procedure allows the link adaptation algorithm tooperate independently of scheduling algorithms. This is beneficial,since such independency makes it easier to design and implement the linkadaptation algorithm.

[0101] CS-4 Must be Used with Caution

[0102] GPRS's RLC/MAC layer has a subtle design flaw that affects itsability to cope with time-varying channels. GPRS's RLC/MAC layer selectsa coding scheme to send the initial transmission of an RLC/MAC block. Ifthe block is received in error, GPRS's RLC/MAC protocol specifies thatthe block must be retransmitted using the coding scheme that was usedfor the initial transmission. The performance of this protocol cansuffer severe degradation when used over time-varying channels.Consider, for example, a scenario in which channel conditions areadequate for CS-4 to be employed. CS-4 only offers acceptableperformance at high C/I's, since errors occurring in CS-4 blocks can bedetected, but not corrected. Suppose sudden degradation in channelquality caused by a change in interference level or attenuation causesthe BLER experienced by CS-4 blocks to increase from a few percent to60%. All blocks received in error must be retransmitted using CS-4,although the block error rate is too high to support the coding scheme.At best, as a result of the sudden drop in airlink quality, datatransmissions will suffer excessive delays. At worst, the TBF will breakafter numerous re-transmissions squander airlink bandwidth.

[0103] Accordingly, the PCU 18 follows a set of constraints governingthe use of CS-4. Uplink and downlink TBFs are never allowed to beginwith CS-4. Furthermore, CS-4 is only used after a period of time haspassed when the PCU 18 was able to observe suitable performance whenusing CS-3. For downlink TBFs, the PCU 18 additionally requires thereceived signal strength reported by the mobile (C_VALUE) and signalvariance (SIGNVAR) fall within suitable ranges set by the systemdesigner. Similarly, for uplink TBFs, the mean signal strength forblocks sent by a mobile (RXLEV) must also exceed a minimum target beforean uplink TBF is commanded to use CS-4.

[0104] Selecting a Code Based on Volume of Data

[0105] A strong case can be made for considering the amount of data tobe carried in the TBF when the amount of data (or amount of remainingdata) to be transmitted is small. Consider, for example, thetransmission of a LLC frame carrying a compressed TCP/IP acknowledgementmessage. Such frames will be only 10-15 bytes long, and will requireonly one airlink block, regardless of which of the four GPRS codingschemes is used. For such small transmissions, it the PCU 18 uses themost robust coding scheme possible.

[0106] Since many of the frames carried are likely to be small, thisscheme can potentially provide a big benefit.

[0107] Coping with A-bis Bandwidth Limitations

[0108] Because of bandwidth limitations for the A-bis interface 14, notall coding schemes can be used on all timeslots allocated to a mobilestation's TBF. The PCU 18 conservatively ensures that the coding schemecommanded by the link adaptation algorithm for a mobile is no weakerthan the weakest coding scheme supported by all timeslots currentlyallocated to it.

[0109] For example, if a multislot mobile is allocated one or moretimeslots with 16 kpbs A-bis links (only CS-1 and CS-2 are supported onthese timeslots) and one or more timeslots with 32 kbps A-bis link (CS-1through CS-4 are supported), the link adaptation algorithm will notselect a coding scheme weaker than CS-2, i.e., only CS-1 and CS-2 may beused by this mobile.

[0110] QuickACK

[0111] The uplink link adaptation algorithm will also employ a featureterm “QuickACK”. At the start of an uplink TBF, the network (e.g., PCU18) must choose an initial coding for each mobile to use. Throughout thelifetime of the uplink TBF, the PCU 18 will periodically send “PacketUplink ACK/NACK” blocks to the mobile containing updated channel codingcommands. It is likely that the mobile station will send a packet uplinkAck/essage each time a tuneable number of radio blocks has beenreceived, N_(poll). (A typical value of N_(poll) is ˜10 radio blocks forGPRS.) At the start of an uplink TBF, it is possible for the network toselect a coding scheme that is too weak to support the desired uplinkchannel performance—e.g, one that gives exceptionally high block errorrates. Oftentimes, such “poor decisions” can be detected within thefirst few radio blocks sent on the uplink TBF. Rather than wait for thefirst N_(poll) blocks to be received to send a packet uplink ACK/NACKmessage with a new channel coding command, it is best to send theACK/NACK message sooner.

[0112] One (bad) way to do this is to make the first polling intervalsmaller than N_(poll). This approach can generate significant messagingoverhead, particularly for TBFs carrying less than N_(poll) radioblocks. The probability that the correct coding scheme is chosen by thelink adaptation algorithm for the mobile at the start of an uplink TBFshould normally be quite high. To prevent squandering downlinkresources, the uplink link adaptation should first determine if it isnecessary to correct the coding scheme being used by the mobile withinthe first few blocks. If it is not necessary, then the algorithmrecommends to the mobile station that the initial “quick” ACK/NACKshould not be sent. In such a case, the mobile station schedules thenext Uplink ACK/NACK at the usual time, i.e., after the first N_(POLL)blocks have been received from the mobile. This judicious sending ofpacket uplink ACK/NACK messages at the start of a TBF is called a“QuickACK.”

[0113] A High-Level Architecture for GPRS Link Adaptation

[0114] To make development and testing go as smoothly as possible overthe lifetime of link adaptation and scheduling features, it is desirableto phase in additional complexity as and when needed. FIGS. 6 and 7illustrate high-level architecture for GPRS link adaptation thatde-couples the selection of a coding scheme based on current linkquality, and the need to use a stronger code based on A-bis restrictionsand remaining data backlog.

[0115]FIG. 6 shows a high-level architecture for the downlink linkadaptation feature. The high-level architecture separates downlink linkadaptation decisions for each mobile with an active downlink TBF intotwo functional blocks at the PCU 18: a downlink CS advisor 40 and adownlink intelligent override 42.

[0116] The downlink CS adviser 40 monitors the quality of a mobilestation's Temporary Block Flow over the TBF's lifetime based onmobile-reported RXQUAL and block errors. When downlink TBFs begin for amobile, the downlink CS adviser 40 looks to see whether there isinformation on channel quality from a previous downlink TBF with themobile that could be used to help determine an initial coding scheme.The downlink CS adviser 40 determines the coding scheme that should beused by selecting the strongest of CS_(BER) and CS_(BLER), independentof A-bis bandwidth limitations or other constraints.

[0117] Although it does not take A-bis bandwidth limitations intoaccount for determining the strongest coding scheme to use, it doesevaluate the maximum rate coding scheme that can be currently used bythe mobile based on A-bis bandwidth limitations. When quality of service(QoS) scheduling is supported, the downlink CS adviser 40 also provideestimates of airlink performance (throughput) under the currentprevailing channel conditions.)

[0118] The downlink intelligent override 42 includes logic to determinewhether it is advantageous (or necessary, because of A-bis limitations)to use a stronger code than the one suggested by the downlink CS adviser40. Namely, the downlink intelligent override 42 contains logic to lookat the amount of remaining data and determine whether a stronger codethan the one suggested by the downlink CS adviser 40 in the same numberof downlink radio blocks, logic to only use a CS that can be supportedby the A-bis capacity currently allocated to timeslots carrying themobile's TB logic to support coding scheme aging and coupling betweenclosed-loop power control and link adaptation (discuss in detail below),and logic to support changing the coding scheme based on the desiredquality of service (QoS).

[0119] The downlink CS adviser 40 is invoked when a downlink TBF startsfor a mobile, any time downlink channel quality reports are receivedfrom the mobile, any time the mobile's downlink TBF is assigned newtimeslots, and any time a TBF ends. It provides information on theweakest coding scheme that can be used successfully for downlinktransmissions.

[0120] The downlink intelligent override 42 is invoked each time adownlink radio block is to be sent to a mobile. It determines the CSthat should be used to encode the current radio block, as well as whichRLC blocks should be packed into the radio block.

[0121] A High-Level Architecture for Uplink Link Adaptation

[0122]FIG. 7 shows a high-level architecture for the uplink linkadaptation feature. As in the architecture of the downlink linkadaptation algorithm, the uplink link adaptation feature is divided intotwo functional blocks at the PCU 18: an uplink CS advisor 50 and anuplink intelligent override 52.

[0123] The uplink CS adviser 50 tracks the quality of a mobile station'suplink Temporary Block Flow over the TBF's lifetime based on RXQUAL andBad Frame Indicator values included in each PCU frame corresponding toan uplink radio block allocated to mobile. When uplink TBFs begin for amobile, it looks to see whether there is information on channel qualityfrom a previous uplink TBF that could be used to help determine aninitial coding scheme. The uplink CS adviser 50 determines the codingscheme that should be used by selecting the strongest of CS_(BER) andCS_(BLER). The uplink CS adviser 50 also provide additional informationneeded for QoS scheduling algorithms.

[0124] The uplink intelligent override 52 includes logic to require thatonly those coding schemes that can be supported by the A-bis capacitycurrently allocated to timeslots carrying the mobile's TBF, logic tosupport coupling between closed-loop power control and link adaptation,and logic to support changing the coding scheme based on the desiredquality of service.

[0125] The uplink CS adviser 50 is invoked when a uplink TBF starts fora mobile, any time a PCU frame corresponding to an uplink blockallocated to the mobile is received by the PCU 18, any time the mobile'suplink TBF is assigned new timeslots, and when the mobile's uplink TBFends. The uplink CS adviser 50 provides information on the weakestcoding scheme that can be used successfully for uplink transmissions tothe mobile for the service being provided over the TBF.

[0126] The uplink intelligent override 52 operates any time a channelcoding command is sent to the mobile—any time the mobile station sends aPacket Uplink ACK/NACK or Packet Uplink Assignment message to themobile.

[0127] Coupling Link Adaptation and Power Control

[0128] A GPRS network has two tools to combat high airlink block errorrates:

[0129] Power: The network can increase transmit power, provided themobile or network are not already transmitting at maximum power levels.

[0130] Code rate: The network can select a coding scheme with a lowercode rate, provided CS-1 is not currently being used over the airlink.

[0131] Link adaptation and power control algorithms are necessarilybased on a common assumption: channel conditions caused by path loss,shadow fading and interference in the near future will be similar tothose observed in the recent past. If sufficient care is not taken,however, power control and link adaptation algorithms can work againstone another. Consider, for example, the painful consequences of adecision by a link adaptation algorithm to jump from CS-1 to CS-3without taking into account the effects of a simultaneous 10 dB drop intransmit power commanded by a power control algorithm.

[0132] Power control algorithms adjust power in attempt to maintainchannel quality within a desired quality range. For example, when biterror rates are lower than the target bit error rate range and themobile is not already using minimum transmit power, the uplink powercontrol algorithm will decrease the mobile's transmit power. When biterror rates are higher than the target bit error rate range and themobile is not already using maximum uplink transmission power, theuplink power control algorithm will command the mobile to increasetransmit power.

[0133]FIG. 8 illustrates the loose coupling between closed-loop powercontrol and link adaptation according to the invention. As shown, thelink adaptation algorithm informs the power control algorithm about theweakest coding scheme which may be used over the airlink in the nextmeasurement interval. The power control algorithm then selects thetarget bit error rate, or equivalently, the target C/I, accordingly.Next, the power control algorithm sets the variable MAX_CS, whichindicates the maximum coding scheme that the link adaptation algorithmshould use in the next measurement interval so that the interferencecaused to neighbor cells is tolerable. This variable depends on thelevel of attenuation being used by the mobile. In general, the higherthe level of attenuation, the larger the value of MAX_CS. Intuitively,if the level of attenuation is high, i.e., the transmit power level islow, then the interference being caused to neighbor cells is also low.At such times, the link adaptation algorithm should be allowed to takeadvantage of coding schemes that require high C/I and yield highthroughput if the prevalent channel conditions permit its usage. If onthe other hand, the transmit power is high, then the interference beingcaused is also high. So, the link adaptation algorithm should berestricted to using coding schemes which require low C/I; this, in turn,would allow the possibility of reducing the transmit power level,thereby reducing interference.

[0134] If pure open-loop uplink power control is applied, or if constanttransmit power is used on the air interface, the link adaptationalgorithm is allowed to use all four GPRS coding schemes.

[0135] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications are intended to be included within the scope of thefollowing claims.

We claim:
 1. A method of adaptively selecting an airlink coding schemein a telecommunications network, comprising: determining a coding schemeoperating region based on a currently used coding scheme andmeasurements representative of a block error rate; and selecting acoding scheme based on the determined coding scheme operating region. 2.The method of claim 1, wherein each of the coding scheme operationregions are predefined operating regions.
 3. The method of claim 1,wherein each coding scheme corresponds to a different coding schemeoperating region; and the selecting step selects the coding schemecorresponding to the determined coding scheme operating region.
 4. Themethod of claim 1, wherein the determining step comprises: estimating ablock error rate for a predetermined one of the coding schemes in eachcoding scheme operating region based on the measurements; comparing theestimated block error rate to expected block error rates for thepredetermined coding scheme in each coding scheme operating region; anddetermining the coding scheme operating region based on results of thecomparing step.
 5. The method of claim 1, wherein the determining stepcomprises: receiving the measurements from a mobile station; generatinga region indicator for each coding scheme operating region based on themeasurements; and determining a coding scheme operating region based onthe generated region indicators.
 6. The method of claim 5, wherein themeasurements include a report of correctly and incorrectly receivedblocks of data.
 7. The method of claim 5, wherein the determining stepfurther comprises: calculating a number of block errors for a number ofblocks sent using the current coding scheme based on the measurements;and wherein the generating step generates the region indicators based onresults of the calculating step.
 8. The method of claim 1, wherein thedetermining step comprises: making the measurements on a signal receivedfrom a mobile station; generating a region indicator for each codingscheme operating region based on the measurements; and determining acoding scheme operating region based on the generated region indicators.9. The method of claim 8, wherein the measurements include a report ofcorrectly and incorrectly received blocks of data and a measure of anumber of bit errors observed in correctly received blocks.
 10. Themethod of claim 1, wherein the determining step determines the codingscheme operating region based on the measurements, which are indicativeof a probability that a number of block errors occurred for a number ofblocks sent using the current coding scheme.
 11. The method of claim 1,further comprising: selecting a set of coding scheme operating regionsfrom multiple sets of coding scheme operating regions based on one ofquality of service and type of service.
 12. The method of claim 1,further comprising: determining a bit error coding scheme based on a biterror rate determined from the measurements; and selecting one of thebit error coding scheme and the block error coding scheme as the codingscheme.
 13. The method of claim 12, wherein the selecting step selectsone of the bit error coding scheme and the block error coding schemehaving a lowest block error rate as the coding scheme.
 14. The method ofclaim 12, wherein the determining a bit error coding scheme step readsthe bit error coding scheme from a look-up table based on themeasurements.
 15. The method of claim 12, wherein the determining stepcomprises: estimating a block error rate for a predetermined one of thecoding schemes in each coding scheme operating region based on themeasurements; comparing the estimated block error rate to expected blockerror rates for the predetermined coding scheme in each coding schemeoperating region; and determining the coding scheme operating regionbased on results of the comparing step.
 16. The method of claim 12,wherein the determining step comprises: generating a region indicatorfor each coding scheme operating region based on the measurements; anddetermining a coding scheme operating region based on the generatedregion indicators.
 17. The method of claim 1, further comprising:changing the selected coding scheme to a first stronger coding schemewhen predetermined criteria regarding performance at a second strongercoding scheme have not been met, the first and second stronger codingschemes being one of same and different coding schemes.
 18. The methodof claim 1, further comprising: changing the selected coding scheme to astrongest coding scheme if an amount of data to be transmitted using theselected coding scheme is less than a threshold amount.
 19. The methodof claim 1, further comprising: changing the selected coding scheme to acoding scheme supported by a transmission time slot when the time slotto be used in transmitting data according to the selected coding schemedoes not support the selected coding scheme.
 20. The method of claim 1,further comprising: performing the method when performance using thecurrent coding scheme falls below a predetermined performance threshold.21. The method of claim 1, further comprising: caching, at an end of afirst downlink communication flow with a mobile station, the selectedcoding scheme; retrieving the cached coding scheme when a seconddownlink communication flow is being established with the mobilestation; and changing the retrieved coding scheme to a stronger codingscheme based on an amount of time between the end of the first downlinkcommunication flow and the beginning of the second downlinkcommunication flow.
 22. The method of claim 1, further comprising:caching, at an end of a first uplink communication flow with a mobilestation, the selected coding scheme; retrieving the cached coding schemewhen a second uplink communication flow is being established with themobile station; and changing the retrieved coding scheme to a strongercoding scheme based on an amount of time between the end of the firstuplink communication flow and the beginning of the second uplinkcommunication flow.
 23. The method of claim 1, further comprising:changing the selected coding scheme based on power control beingperformed by a power control algorithm.
 24. A method of adaptivelyselecting an airlink coding scheme in a telecommunications network,comprising: estimating a block error rate for a predetermined one of thecoding schemes in each coding scheme operating region based onmeasurements representative of block error rate; comparing the estimatedblock error rate to expected block error rates for the predeterminedcoding scheme in each coding scheme operating region; and determining acoding scheme based on results of the comparing step.
 25. The method ofclaim 24, wherein the determining step determines the coding schemeassociated with the coding scheme operating region having the estimatedblock error rate less than the expected block error rate as thedetermined coding scheme.
 26. The method of claim 25, wherein thedetermining step determines the coding scheme associated with the codingscheme operating region having a smallest difference between estimatedand expected block errors and having the estimated block error rate lessthan the expected block error rate as the determined coding scheme. 27.A method of adaptively selecting an airlink coding scheme in atelecommunications network, comprising: generating a region indicatorfor each coding scheme operating region based on measurementsrepresentative of block error rate; and determining a coding schemeoperating region based on the generated region indicators.
 28. Themethod of claim 27, wherein the measurements include the measurementsinclude a report of correctly and incorrectly received blocks of data.29. The method of claim 27, further comprising: calculating a number ofblock errors for a number of blocks sent using the current coding schemebased on the measurements; and wherein the generating step generates theregion indicators based on results of the calculating step.
 30. Themethod of claim 27, wherein the region indicators are indicative of aprobability that a number of block errors occurred for a number ofblocks sent using the current coding scheme.
 31. A method of adaptivelyselecting an airlink coding scheme in a telecommunications network,comprising: determining a first coding scheme based on measurementsrepresentative of one or more conditional channel quality metrics;determining a second coding scheme based on measurements representativeof a block error rate; and selecting one of the first and second codingschemes as the coding scheme.
 32. The method of claim 31, wherein theselecting step selects a stronger one of the first and second codingschemes as the coding scheme.
 33. The method of claim 31, wherein theconditional channel quality metric is bit error rate.
 34. The method ofclaim 31, wherein the selecting step selects one of the first and secondcoding schemes based on a block error rate.
 35. An apparatus foradaptively selecting an airlink coding scheme in a telecommunicationsnetwork, comprising: an advisor unit determining a coding scheme basedon measurements representative of block error rate; and an override unitselectively changing the determined coding scheme based on at least oneof an amount of data to be transmitted, a bandwidth of an interface inthe telecommunications network, a time period since the measurementswere one of made or received, a power control operation, and a desiredquality of service.